Within the prior art, various means have been developed to counter vibration problems. These include passive Tuned Vibration Absorbers (TVAs), Adaptive TVAs (ATVAs), Active Structural Control (ASC), and Active Isolation Control (AIC), all of which will be briefly described herein.
Passive Tuned Vibrations Absorbers (TVA's) are known devices which find utility in absorbing low-frequency vibration by providing localized vibration reduction at their attachment point. Although, TVAs are generally well adapted for attenuating low-frequency noise, they are generally somewhat limited in range and effectiveness, that is, they are only effective at a particular frequency (fn) or within a narrow frequency range. Therefore, TVAs may be ineffective when the disturbance frequency changes, such that the TVA is not any longer excited at its resonant frequency (fn). Moreover, passive TVAs may be unable to generate proper magnitude or phasing of forces needed to effectively suppress vibration. Furthermore, at low frequencies, TVAs are required to be relatively massive for adequate vibration reduction.
When a wider range of vibration cancellation is needed, various Adaptive TVAs (ATVAs) may be employed. For example, U.S. Pat. No. 3,487,888 to Adams et al. entitled "Cabin Engine Sound Suppresser" teaches an ATVA where the resonant frequency (fn) can be adaptively adjusted by changing the "length" of a beam, or the "rigidity" of a resilient cushioning material. Although, the range of vibration attenuation may be increased with ATVAs, they still may be somewhat ineffective for certain applications, in that their range of adjustment may not be broad enough, or they may not be able to generate large enough forces to effectively control vibration.
Where a higher level of noise and/or vibration attenuation is desired, or multiple vibration frequencies need to be addressed, actively controlled vibration systems may be employed. One particular class of active systems are termed Active Isolation Control (AIC) systems. AIC systems include "active mountings" which are attached between a disturbance source (ex. engine) and its attachment structure (ex. frame, etc.). Active mountings include an actively driven element (actuator, inertial shaker, or the like) which provides appropriately phased active control forces of the proper magnitude and frequency for preventing vibration transmission from disturbance source into the attachment structure. Known AIC systems include the feedforward-type, in which a reference signal(s) from a reference sensor(s) is used to provide a signal(s) representative of a disturbance(s) to a control process. Likewise, error sensors provide error signals indicative of the residual vibration or noise. These reference and error signals are processed by a digital controller to generate output signal(s) of the appropriate phase, magnitude, and frequency, i.e., the anti-vibration signal(s). These anti-vibration signal(s) drive the active mounting(s) to reduce vibration transmission from the disturbance source to structure, thereby resultantly controlling structural vibration or noise at some remote location.
In some applications there may be insufficient space envelope to house the active elements (actuators, etc.) within the mounting. Further, there may be alternate vibration paths into the structure, or the appropriate actuation directions required for good vibration attenuation may be difficult to achieve within the space constraints of a mounting. Therefore, under these circumstances, other types of active vibration control may be implemented, such as Active Structural Control (ASC).
ASC systems utilize Active Vibration Absorbers (AVAs) or Inertial shakers which are actively driven along their acting axes at the appropriate frequency, amplitude, and phase to generate counter inertial control forces at their attachment points. By providing the appropriate frequency, phasing, and amplitude to the control forces, vibration at the attachment point or elsewhere may be controlled. ASC systems may also control acoustic noise at remote locations.
In various processes where rolls, such as calender rolls are utilized, such as in the paper making process, there may be dynamic vibrations of one or more of the calender rolls in the calendar. The term "calender", as it is used herein, refers to any system of two or more rolls through which a medium is passed. Vibration of such rolls may cause defects in quality of the calendered medium which may be objectionable. For example, variations in the thickness along the length of the calendered medium may occur. It also may be the case that the speed of the calendering process may have to be slowed to correct these defects. Simply, it may be desirable to improve the quality of the calendered product. Heretofore, no systems have addressed the problems of variations in thickness of the calendered product due to dynamic vibration of the rolls.